The rod cell, a small form of Candida albicans, possesses superior fitness to the host gut and adaptation to commensalism

Abstract

Candida albicans deploys various morphological forms through complex switching mechanisms, ensuring its survival and thriving as a commensal or pathogen in vastly different human niches. In this study, we demonstrate that a novel ''rod'' morphological form of C. albicans coexists and is interchangeable with previously reported white, gray, and opaque forms, constituting a tetra-stable phenotypic switching system. Rod cells arise from the efg1 mutant of SC5314 cells or from the clinical BJ1097 strain cultured under glucose-free conditions. They are characterized by a distinct gene expression profile and can be stably maintained through in vitro passaging or in vivo inhabitation of the gastrointestinal (GI) tract of mice. Remarkably, the majority of the efg1 mutant cells become rod cells in N-acetylglucosamine (GlcNAc)-containing medium, and the GlcNAc sensor Ngs1 is instrumental in converting the white or gray cells to the rod cells. Conversely, glucose inhibits rod cells through Cph1; consequently, the loss of Cph1 in the efg1 mutant cells permits their conversion to rod cells in glucose-replete media. Notably, rod cells of the efg1/cph1 mutant display superior adaptation and longer persistence in the murine GI environment than wild-type white cells. Taken together, these findings establish rod cells as a previously unappreciated form that is not only morphologically and transcriptionally distinguishable but also defined by specific genetic and environmental determinants, shedding light on complex fungus-host interactions.

1. Introduction

Candida albicans is a commensal resident in the gut of healthy humans but can become pathogenic in immunocompromised individuals. Central to its transition from commensalism to pathogenesis is the transition between various morphological forms, including white-opaque switching, white-GUT transition, and white-gray-opaque switching [1–5]. The white, gray, and opaque phenotypes manifest unique cellular and colonial morphologies. White cells are characterized by their relatively small volumes and round shapes, forming hemispherical colonies with a white coloration. Opaque cells display an elongated morphology and larger sizes, resulting in flatter, rough, and dark-colored colonies [1–6]. Gray cells are smaller than opaque cells, forming dark-colored colonies with a smooth surface [5].

The transcription factors Efg1 and Wor1 play pivotal roles in white-opaque switching [7–14]. Liang et al. [15] and Park et al. [16–18] reported that the loss of EFG1 function activated white-to-gray phenotypic switching in both mating-type locus (MTL) heterozygous strains and MTL homozygous strains. Notably, hemizygosity and mutation of EFG1 in clinical isolates license this phenotypic transition in C. albicans cells [15–18]. Deletion of EFG1 and WOR1 in the clinical C. albicans strain BJ1097 stabilizes cells in the gray phase [5]. We previously reported an opaque-like phenotype in the efg1/wor1 mutant regulated by ambient pH [19]. However, this opaque-like form was subsequently classified as a 'gray' phenotype by Park et al. [16]. Intriguingly, the 'gray cells' reported from different laboratories exhibit tremendous heterogeneity in cell size, varying from strain to strain and from cell to cell. In Huang's laboratory, the clinical C. albicans isolate BJ1097 displayed a broad range of gray cell sizes in response to various growth conditions [5]. In Bennett's laboratory, an 'INT' cell state (intermediate) of EFG1 heterozygous cells was reported [15]. Due to the phenotypical similarity between INT and gray states, they referred to the INT state as the gray state. In Soll's laboratory, two subgroups of the gray phenotype including 'tiny elongate' and 'transition' were noted [18]. In our laboratory, we also observed two distinct types of gray cells differing in size (big and small) when cultured in different media under various growth conditions. It is unknown whether small-sized 'gray' cells can be maintained in high purity under specific conditions and whether they have the capability to transition into other established cell types.

In this study, we designate the smaller cell state as the 'rod' state, as these smaller cells exhibit phenotypic similarities to bacilli (rod-like bacteria) in both cell shape and size. We uncovered that the carbon source plays a pivotal role in the formation of rod cells, and we successfully obtained rod cells in a high purity form from efg1-null mutants under specific culture conditions. Our result...